Question

which factor is a major contributor to outdoor air pollution?
radon
tossil fuels
ultrafine particles
carbon monoxide

Explanation:

Fossil fuels such as coal, oil, and gas are widely burned in power - generation, transportation, etc. Their combustion releases a large number of pollutants like sulfur dioxide, nitrogen oxides, particulate matter, etc. into the outdoor air, contributing significantly to outdoor air pollution. Radon is more of an indoor air pollutant. Ultrafine particles are a type of pollutant but are often the result of other pollution - causing activities. Carbon monoxide is a pollutant but is mainly a by - product of incomplete combustion and not as major a contributor to outdoor air pollution as fossil fuels in terms of overall impact.

Answer:

B. fossil fuels